The development of a non-cooperative airborne obstacle detection radar function for aircraft, in particular for drones, is essential in order to enable the insertion of autopiloted aircraft into unsegregated airspace. It participates in the obstacle detection and avoidance function known under the term “Sense and Avoid”.
Such a radar system must be capable of a very wide field of observation, typically +/−110° in azimuth and +/−15° in elevation, and must be capable of scanning the airspace in a very short time, in view of the time needed in order to engage an avoidance manoeuvre in the case of a collision risk. These characteristics correspond approximately to the environment observational capacity of a “human” pilot.
For reasons of total penetration range in rainy weather, of availability of low-cost microwave components, and of ease of integration onto the carrier, such a radar system advantageously uses X band.
For such an application, it is advantageous to use one or more wide-field transmission antennas, and to simultaneously form multiple reception beams within the illuminated field. This solution is conventionally implemented by means of antenna arrays whose radiation patterns must have a sufficient directivity to localize the targets with a high enough precision. This directivity is typically better than 10° in both planes. In addition, the antenna radiation patterns must have the lowest possible levels of secondary lobes in order to reject the ground clutter, in particular during low-altitude flight phases. Furthermore, the surface area of the antenna must be large enough to cover the required total range with a reasonable power level which is, generally speaking, of the order of 20 watts. In addition to these technical constraints, the radar system must be able to be installed on various types of aircraft, and the constraints on volume for the electronics and surface area available for the antenna are very tight. Lastly, the overall cost of the electronics must be minimized.
The challenge is thus to define a radar antenna architecture and an associated processing system allowing high-quality radiation patterns to be obtained, while at the same time minimizing the volume of the electronics and the antenna surface area to be installed.
The primary objectives to be taken into account in the definition of such a radar system are notably the following:                Obtain a wide instantaneous coverage of the field of observation by means of one or more wide transmission beams, in association with the formation of reception beams;        Facilitate the integration by minimizing the surface area of the antenna while at the same time maintaining the required range and keeping to reasonable transmission power levels, for example within the 20 watt class;        Ensure a directivity that is sufficient for separating targets and for reducing the ground clutter returns in the main lobe, for example within the 10° class or less;        Minimize the secondary lobe levels in order to limit ground returns as far as possible;        Minimize the number of transmission and reception channels in order to reduce the cost of the device;        Choose a flexible architecture capable of supporting modifications to the specifications.        
In cases where the same types of problems and issues are posed, electronic scanning or transmission beam switching techniques associated with reception beam formation by computation are generally implemented, by using array antennas, active or otherwise. Unfortunately, in order to guarantee an unambiguous spatial sampling over a wide area, the elementary sources forming the array must be separated from one another by a fraction of a wavelength. Considering an antenna with a 10° aperture in both planes, the number of channels is thus of the order of 100, which cannot be envisaged for an application of the “sense and avoid” type, for reasons of cost and complexity. Furthermore, such a solution would commandeer a continuous installation surface area of around 20 cm by 20 cm per antenna panel, which is not compatible with all carriers, especially as two panels are needed in order to cover the entire azimuthal field over an angular range of +/−110°.
It would be possible to use open hole arrays, but in view of the demands on the level of the antenna secondary lobes, the number of channels would remain very high, typically of the order of 50. Furthermore, this solution would not allow an easier integration onto the carrier, given that the continuous surface area taken up remains unchanged for the same antenna aperture.
Alternatively, it would be possible to use a multiple input/output access antenna array, of the MIMO type, associated with a colored emission. The principles of colored emission are notably described in the article by Francois Le Chevalier: “Space-time transmission and coding for airborne radars” published in Radar Science and Technology, Volume 6, December 2008. However, this type of device presents the following drawbacks:                Since the transmission array simultaneously covers a wide range in elevation, it is necessary to supply each of the sources of the transmission array with a coded signal, where the codes need to all be orthogonal to one another. The electronics needed and the associated processing are therefore complex, and become more so as the size of the array increases;        The implementation of these codes is carried out to the detriment of the distance resolution, which is a problem in particular when the detection of the target has to be carried out in contrast with respect to the ground clutter.        